Off-line paper formation sensors are known in the art. For example, the N.U.I. paper formation sensor, manufactured by NORAM Quality Control and Research Equipment Ltd. scans one spot at a time and can be used to measure paper mass variation. The Quebec North Shore Mead formation tester, manufactured by Electronic Associates, also scanned one spot at a time and gave wavelength power spectra. The Paprican Microscanner system, developed by the inventors herein and manufactured by NORAM Quality Control and Research Equipment Ltd., acquires a two-dimensional image and measures the mass variation, graininess as well as floc size histogram.
On-line paper formation sensors are also known in the art. Thus, the Measurex formation sensor, manufactured by Measurex, scans one spot at a time and gives mass variation measurements and floc size histogram. The Sentrol formation sensor also scans a single spot and extracts hard-clipped auto-correlation function as formation measurements. The Formspec formation analysis system, manufactured by Albany International, uses the visible light and a charge-coupled device (CCD) linear array. Because it looks at the whole width of the paper roll at a time, the whole paper formation variation along the width can be acquired quickly at the cost of low resolution.
The Intec system, manufactured by Intec Corp., also looks at the whole width of the paper roll at the same time but with a laser light source and a fiber optic receiver. It has the same advantages as well as disadvantages as the Formspec system. It is also designed to detect paper defects such as breaks, streaks and clustered or repeated holes. Accuray formation analysis system also scans the whole width of the paper roll at the same time along the cross direction using a linear array detector. It accumulates data of several scans, and an opacity variation coefficient, corresponding to each array element, is measured along the machine direction. Mass variation is obtained from the average of all of the opacity variation coefficients. The change in opacity variation coefficients along the cross direction gives some indications of cross direction uniformity. All of these on-line sensors are not capable of measuring paper anisotropy and of detecting pinholes accurately. Power spectra measurements, proven to be very useful with off-line systems, are also not addressed with any of the on-line systems.
Patents which contain teachings relating to paper formation sensors include U.S. Pat. No. 4,644,174, Ouellette et al, U.S. Pat. No. 4,019,819, Lodzinski and U.S. Pat. No. 3,196,072, Wirtz. The Ouellete et al patent teaches an application of analog filters to the spectral analysis of the transmitted profile of the formation. It neither uses detector arrays nor is it sensitive to machine direction/cross-machine direction variations. The Lodzinski patent teaches the measurement of optical properties, including opacity and Kubelka-Munk coefficients which are measured with on-line sensors in accordance with the teachings in this patent. However, formation measurement is not included in this patent.
The teachings in the Wirtz patent seem to be predicated on the idea that control of the space density of pulp fibers will allow control of the paper making process. A pressure replicate of the surface of the web is formed by pressing a plastic film against the surface with a roller and the plastic is then viewed in transillumination by an optical. detector which counts the number of "fibers" per inch. This patent is of interest having regards to the present application in that it teaches the use of a reciprocating sensing head illustrated in FIG. 3 thereof. However, the sensing head does not have two separate linear arrays, nor does it have a beam splitter to split the light in the direction of both arrays. In addition, it does not obtain values for the same paper descriptors as the inventive system does.